1. Field of the Invention
The present invention relates to a cylinder head for an internal combustion engine, and particularly to the improvements of a cylinder head structure for a spark-ignited internal combustion engine capable of more compactly mounting related parts, such as intake and exhaust valves, a spark plug, and engine valve operating units, in place.
2. Description of the Prior Art
As is generally known, the installation position of a spark plug on an engine cylinder head (relating to the shape of a combustion chamber) plays a very important part in a combustion quality. For maximizing engine performance, a pent-roof type is often used as a typical combustion-chamber shape. In such high-performance engines with pent-roof combustion chambers, a spark plug is generally located substantially in the center of the pent-roof combustion chamber, for shortening a flame propagation distance and thus promoting good combustion. Also, on engines with pent-roof combustion chambers, a spark-plug hole bored or drilled in an engine cylinder head for spark-plug installation/removal, is usually formed as a straight through-opening which communicates the substantially center of the pent-roof combustion chamber and is drilled downwards from the upper part of the engine. However, in such engines having straight spark-plug holes, engine-valve's related parts mounted on the cylinder head are restricted in layout due to the necessity for providing a desired thickness between the spark-plug hole and each of the valve's related parts. To put it concretely, the installation position of the valve operating unit, which operates intake and/or exhaust valves, is limited by the straight spark-plug-holes drilled in the cylinder head. The engine valve must be arranged in due consideration of the shape of the combustion chamber. Also, it is desirable to produce an optimal gas flow (of less energy loss) both on intake stroke and on exhaust stroke. For the reasons mentioned above, the inclination and size (particularly, the size of valve head) of intake valve is designed to be usually different from those of exhaust valve. Usually, the inclination of intake valve is steeper than that of exhaust valve. Thus, if the spark plug is arranged in the cylinder head in a manner so as to satisfy the combustion quality as discussed above, respective valve operating units related to intake and exhaust valves would be arranged in the cylinder head far away from the combustion chamber, owing to the design difference between the intake and exhaust valves. In recent years, there have been proposed and developed various automotive electromagnetically-powered valve operating apparatus each having electromagnetically-operated valve units for electromagnetically opening and closing intake and exhaust valves. On engines with electromagnetically-powered valve operating apparatus, there is an increased tendency for valve units to be arranged far away from the combustion chamber. Commonly, a combination of the electromagnetically-powered valve operating apparatus and the previously-noted straight spark-plug hole opening to the combustion chamber, results in an increased cylinder-head overall height (consequently, an increased engine overall height). This is contrary to demands for lightweight, small-sizing, and reduced engine production costs.